Treat empty filters {} the same as None.

[bdice]

Description

I have been running benchmarks for synced collections (#484) and noticed that my project was loading state point data even when not requested.

I traced this down to the recent changes in #332 that changed the default filter behavior from None to {}. This prevented some of the "fast path" checks from triggering, and caused a full index for the project to be constructed.

Motivation and Context

Bug fix.

I don't know how to test this effectively, because it only affects internal APIs and performance, not correctness.

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog and added all related issue and pull request numbers for future reference (if applicable). See example below.

[codecov]

Codecov Report

Merging #514 (f217886) into master (ffcc61a) will increase coverage by 0.15%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
+ Coverage   76.91%   77.06%   +0.15%     
==========================================
  Files          42       42              
  Lines        5830     5835       +5     
  Branches     1152     1154       +2     
==========================================
+ Hits         4484     4497      +13     
+ Misses       1057     1051       -6     
+ Partials      289      287       -2     

Impacted Files	Coverage Δ
signac/contrib/project.py	85.08% <50.00%> (+0.60%)	⬆️
signac/__main__.py	71.89% <0.00%> (+0.44%)	⬆️

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[vyasr]

I'm fine with not testing this for now.I didn't notice that the innocuous-looking call to project._find_job_ids in JobsCursor.__iter__ that resulting from filter=None would be the difference between slow and fast iteration, so in lieu of specific testing I'm going to add some documentation of this behavior in a few places so that it's easier for developers to be aware.